1. Field of the Invention
The invention relates to a mixed-flow type water jet pump to be used as a propulsion unit of a watercraft.
2. Description of the Related Art
A watercraft which is propelled by a water jet pump, for example, a personal watercraft (which is also referred to as a "PWC") is configured so as to suck water (including sea water) through a water intake port provided on a bottom of a hull of the watercraft, to pressurize and to accelerate the water by the water jet pump and to eject the water rearward from the hull through an outlet port of the water jet pump, thereby advancing the watercraft.
The water jet pump is configured so that an impeller is rotatably provided in a casing of which inner periphery is a circular cylinder-shaped.
Water jet pumps are broadly divided into so-called "mixed-flow type" water jet pumps and a so-called "axial-flow type" water jet pumps. In a "mixed-flow type" water jet pump, the outside diameter (outer edge) of a blade of an impeller thereof, and the outer diameter of a base portion thereof having a circular section to which the blade is attached, and the inner diameter of a inner wall of a pump casing thereof for covering the impeller, are gradually increased from the intake side toward the pressurization side. In an "axial-flow type" water jet pump, the outside diameter (outer edge) of an impeller, the outer diameter of a base portion having a circular section to which the blade is attached, and the inner diameter of a inner wall of a pump casing for covering the impeller, are straight (constant) from the intake side to the pressurization side.
In the water jet pump, pump performance is affected very greatly by the size of a tip clearance between the inner wall of the pump casing and the outer edge of the blade of the impeller. More specifically, the pump performance is degraded if the tip clearance is too great, and there is a possibility that a tip (an outer peripheral end) of the blade might temporarily come in contact with an inner wall of the pump casing if the clearance is too small. For this reason, generally, the tip clearance between the inner wall of the pump casing and the outer edge of the blade of the impeller is set to about 0.20 mm to about 0.30 mm.
In a mixed-flow type water jet pump, if the axial position of an impeller in a casing of the water jet pump is changed in a longitudinal (axial) direction, the tip clearance between the inner wall of the pump casing and the outer edge of the blade of the impeller is varied.
When assembly is performed, adjustment of the axial position of the impeller with respect to the pump casing is performed in the following way, such that the tip clearance between the inner wall of the casing and the outer edge of the blade of the impeller can be kept within the above-mentioned predetermined value.
As shown in FIG. 8, the pump casing 1C of the mixed-flow type water jet pump P is divided into two portions, that is, a "front casing 1Cf" and a "rear casing 1Cr" in a portion in which the pump casing 1C has the greatest diameter. A pump shaft 1S is rotatably provided in the rear casing 1Cr through a bearing 1H such that a front end thereof is protruded forward. An impeller 1Z to be rotated integrally with the pump shaft 1S is attached on the front end of the pump shaft 1S.
The impeller 1Z is installed as follows. A suitable number of sheet-shaped shims 35 for adjusting the clearance are provided between the bearing support side of the pump shaft 1S and the impeller 1Z, such that a clearance between the inner wall of the pump casing 1C (front casing 1Cf) which is a slanted face and the outer peripheral edge of the impeller 1Z is set within a predetermined range. Then, a female screw hole on the rear end of the impeller 1Z is screwed and fixed to a male screw on the front end of the pump shaft 1S.
When the impeller 1Z is completely arranged as described above, the front casing 1Cf is attached to the rear casing 1Cr having the pump shaft 1S and the impeller 1Z provided therein, and both of them are fixed to each other with attaching bolts (not shown).
When the assembly of the water jet pump is thus completed, a clearance gauge is inserted from the outside (the outside of the pump intake side) into the tip clearance between the inner wall of the front casing 1Cf and the outer peripheral edge of the impeller 1Z, thereby ascertaining whether the tip clearance is within a predetermined range. After an inspection of the clearance, the water jet pump is mounted on a personal watercraft, at the same time inserting and fixing a spline shaft of a propeller shaft (not shown) coupled to the engine at an opposite side, into a spline hole 1b formed on the front end of the impeller.
However, if the clearance is not obtained within the predetermined range during the inspection performed by using the clearance gauge, the front casing 1Cf is removed from the rear casing 1Cr by loosening the attaching bolts. The impeller is, further, removed from the pump shaft. The number of the shims is increased or decreased, and the water jet pump is assembled again, followed by checking and confirming the clearance again.
Typically, assembly and disassembly of the water jet pump is performed many times to adjust the clearance. Moreover, a curved clearance is measured by means of the clearance gauge plate. Therefore, the clearance cannot be accurately measured due to a rigidity of a plate (tongue) shaped "clearance gauge" which is generally used. Consequently, the assembly and disassembly could be performed many more times.
U.S. Pat. No. 4,541,808 (Japanese Patent Publication No. Sho 60-30599) describe a mixed-flow type water jet pump according to the prior art, and Japanese Patent Publication No. Hei 3-11955 and Japanese Laid-open Patent Publication No. Hei 9-207885 describe an axial-flow type water jet pump according to the prior art.